The present invention relates to a cutting valve of a valve assembly particularly of a transversing incore probe system (called hereinlater TIP) for calibrating an output area neutron detector disposed in, for example, a boiling water reactor (BWR) and adapted to monitor an incore neutron flux in a reactor core.
FIG. 2 is a view showing an entire structure of the TIP to be disposed in the BWR, and with reference to FIG. 2, in the BWR, there are arranged a plurality of output area neutron detector assemblies 2 disposed to predetermined portions in a reactor pressure vessel (RPV) 1, calibrating tubes 3 disposed in these neutron detector assemblies 2, detector guide tubes 4 connected to lower ends of the calibrating tubes 3, indexing devices 5 connected to the detector guide tubes 4, isolation valve units 8 including ball valve units 8a and cutting valves 8b and connected to outside portions of a reactor containment vessel 7 through a flanged portion 6, shielding vessels 9 connected to the isolation valve units 8 through the detector guide tubes 4, and detector drive devices 10 connected to the shielding vessels 9 through the guide tubes 4.
Accordingly, the TIP detector advances in the detector guide tube 4 by a distance corresponding to a feed-out detector cable length. When the TIP detector is fed out from the shielding vessel 9, the ball valve 8a of the isolation valve unit 8 is opened, the TIP detector is then guided into the reactor containment vessel 7 through the isolation valve unit 8 and the penetrating flange 6 and, finally, stops at the core top portion of the calibration tube 3. Thereafter, the TIP detector is pulled out for measuring the neutron flux in the RPV 1.
Incidentally, when any abnormal condition occurs in the reactor, an isolation signal is generated to close all the isolation valve units 8 in the reactor containment vessel 7. However, in the state that the detector cable passes through the ball valve 8a, the ball valve 8a cannot be closed.
In such state, a cutting blade mounted to the cutting valve 8b is operated to cut the detector cable passing the cutting valve 8b, and simultaneously, the cutting valve 8b is sealed by the cutting blade to thereby isolate and seal the penetrating portion of the TIP system.
FIG. 5 shows a structure of one conventional cutting valve 8b, in which a cylinder 12 is disposed in a valve box 11 and a piston 14, to which a cutting blade 13 is mounted to be directed downward, as viewed, is accommodated in the cylinder 12, and a powder plug 16 in which an explosive powder 15 is filled up is disposed to an opening of the cylinder at a portion above the piston 14.
A valve seat 21 is disposed below the cylinder 12, and a through hole 17 through which the detector guide tube 4 passes is formed below the cylinder 12 so as to extend in a direction normal to the cutting blade 13. The detector guide tube 4 is inserted into the through hole 17 and the outer peripheral side of the guide tube 4 are fixed to the through hole 17 by mounting a guide tube inlet plug 18 and a guide tube outlet plug 19 to the valve box 11 and the valve seat 21 at inlet and outlet sides of the through hole 17, respectively. Thus, it may be said that the valve box 11 serves also as a holder for holding the detector guide tube 4. A detector cable 20 is inserted into the inside portion of the detector guide tube 4.
In the structure of the cutting valve described above, when the cutting blade 13 is driven, the powder 15 is exploded by current conduction to a coil, not shown, disposed in the powder 15 in the powder plug 16 to cause an explosive pressure, by which the piston 14 is slid downward in the cylinder 12 and the cutting blade 13 is driven and moved towards the detector guide tube 4, whereby both the detector guide tube 4 and the detector cable 20 inserted therein are cut together. In this operation, the front end of the cutting blade 13 abuts against the valve seat 21 in a manner that the side portion of the cutting blade 13 closes the through hole 17.
The cutting blade 13 is formed of a material having high hardness such as hot tool steel (SKD-12) for cutting both the detector guide tube 4 formed of a stainless steel and the detector cable 20 formed of a piano wire.
By the way, when a reactor accident occurs, a temperature inside the reactor containment vessel rises to about 171.degree. C., and at the same time, a temperature of the cutting valve 8b mounted to a portion near the penetrating flange 6 of the reactor containment vessel 7 also rises to near the same temperature. For this reason, the cutting blade 13 is formed of the hot tool steel having a large hardness such as SKD-12 for cutting both the detector guide tube 4 formed of a stainless steel and the detector cable 20 formed of a piano wire even if the hardness of the cutting blade 13 is lowered by such high temperature.
In usual, an operation test has been performed by exploding the powder 15 for confirming the function of the cutting valve 8b once in several years. However, the operation test has been carried out under an ordinary, i.e. room, temperature, the cutting blade 13 may be broken because of its hardness and the sealing performance thereof may be lowered. Accordingly, in order to prevent the cutting blade 13 from breaking, it is necessary in the operation test to create an atmosphere having substantially the same temperature as that in the reactor containment vessel 7 at which an accident will occur in the reactor by disposing a high temperature tank or the like in a reactor power plant.
As discussed above, in order to carry out the operation test for the conventional cutting valve 8b, it is necessary to convey the high temperature tank in the reactor power plant, requiring much labor, and moreover, in such operation test, workers must perform the operation test under the severe high temperature environment, providing a troublesome problem.